Professor Heather Brooke, of City, University of London, warned would make it significantly more difficult to secure the public release of material.

She said: “The freedom of information law sets a default for openness but with these documents, the government instead set the default at ‘secret’ and only in a few exceptions will the public get a look.

“Negotiating a trade policy is sensitive. No one doubts that. But there must be transparency and public engagement if people are to have confidence that the policy works for all and not just an elite few.”

Existing freedom of information rules contain powers for ministers to withhold material to protect relations with a foreign state, including the conduct of negotiations.

But campaigners said the DIT was seeking excessive control, including by apparently trying to limit American transparency rules.

Earlier this year, information released by Washington caused embarrassment in London by revealing that a 27-strong British delegation sent for inaugural discussions contained no experienced trade deal negotiators.

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